1. Field of the Invention
The present invention relates to a cross roller bearing device that may receive a load in universal directions with a single bearing.
2. Description of the Related Art
In a conventionally well known conical roller bearings (see FIG. 15), a rotary centerline of each conical roller is slanted so that a load both in an axial direction and in a radial direction may be received. However, the axial load that may be received by the conical roller bearing is limited to one direction (a load from a back side). Accordingly, in order to make it possible to receive both in a first axial direction indicated by an arrow A and in a second axial direction indicated by an arrow B in FIG. 15, it is necessary to use the two conical roller bearings in face-to-face relation as shown in FIG. 16. (In case of FIG. 16, since front surfaces of the bearings are caused to face each other, this is a front surface combination bearing. Inversely, the back-to-back combination bearing is referred to as a back surface combination bearing.) This causes defects in space efficiency and in weight increase.
Accordingly, there is provided a cross roller bearing device in which loads in the first and second axial directions and in a radial direction may be received by a single bearing. FIG. 17 shows the cross roller bearing 1. FIG. 18 is a cross-sectional view showing a portion taken along the line D--D of FIG. 17. In the cross roller bearing, the rollers 2 are alternatively disposed on the same orbit so that centerlines C.sub.1 and C.sub.2 of the adjacent rollers 2 are arranged to intersect with each other, and the rollers 2 are clamped by an inner race 3 and an outer race 4. Orbital surfaces 3a, 3b, 4a and 4b that form angles of 45 relative to the centerlines C are formed in the inner race 3 and the outer race 4, respectively. The rolling surfaces of the rollers 2 with the centerlines intersecting with each other are brought into contact with a pair of orbital surfaces 3a and 4a or a pair of orbital surfaces 3b and 4b.
A retainer 5 is disposed between the respective adjacent rollers 2. As shown in FIG. 19, the retainer 5 has contact surfaces 5a and 5b having substantially the same inner diameter as the roller diameter so that the retainer 5 may be in surface contact with the rolling surfaces of the adjacent rollers 2. The centerlines C.sub.1 and C.sub.2 of the contact surfaces 5a and 5b are arranged to intersect with each other. Then, each roller 2 may rotate while being positioned within the orbit of the bearing and keeping the suitable contact condition with the orbital surfaces, by the contact surfaces 5a and 5b. The above-described conventional cross rollers are described in detail in Japanese Utility Model Publication No. Hei 2-17214 and Japanese Utility Model Publication No. Hei 5-11379, for instance.
However, the conventional cross rollers suffer from the following disadvantages. The explanation will be given in conjunction with FIG. 18. With respect to the roller that rotates with the rolling surface 2a in contact with the orbital surfaces 3a and 4a, the orbital surfaces 3b and 4b serves as guide surfaces for supporting the end faces 2b of the roller 2. However, with respect to the roller located in front of or behind this roller 2, its rolling surface 2a is in contact with the orbital surfaces 3b and 4b, and the orbital surfaces 3a and 4b serve as guide surfaces for supporting the end faces 2b. Namely, each orbital surfaces must have the same diameter and length of the roller in order to work as the guide surfaces as well. Also, its cross-sectional shape should be the same even in any direction of the centerline of the roller. For this reason, it is impossible to use the conical roller as the roller of the cross roller bearing.
By the way, each orbital surface forms an angle of 45.degree. with respect to the centerline of the bearing. Accordingly, the circumferential length of the portion P.sub.1 is different from that of the portion P.sub.2. (The circumferential length through P.sub.2 is longer than that through the portion P.sub.1.) However, the roller 2 which is in contact with each orbital surface which is different in circumferential length in places has the constant diameter. A slippage would be caused between the roller 2 and the respective orbital surfaces. In order to enhance the rotational precision of the bearing, it is necessary to impart a pre-pressure to the roller by the inner race and the outer race to some extent. However, since the frictional heat is generated by the above-described slippage, it is necessary to limit the application of the pre-pressure to some extent for avoiding the thermal sticking. Also, for the same reason, the bearing is not suitable for the high speed rotation.